1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use in an image forming apparatus such as a copying machine or a printer, and a heater adapted for use in such image heating apparatus.
2. Related Background Art
For heat fixing apparatus, there have been employed the heat roller fixing method based on contact heating satisfactory in heat efficiency and safety, and the film heating method capable of energy saving.
The heat fixing apparatus of the heat roller fixing type is basically composed of a heating roller (fixing roller) serving as a heating rotary member and an elastic pressure roller maintained in pressure contact therewith and serving as a pressurizing rotary member. Such paired rollers are rotated, and a recording material (a transfer material sheet, an electrostatic recording sheet, an electrofax paper or a printing sheet or the like) bearing an unfixed image (toner image) is introduced into and pinched, converged and passed by the nip of the paired rollers, whereby the unfixed image is fixed with heat and pressure as a permanent image on the recording material by the heat from the heating roller and the pressure from the elastic pressure roller at the nip.
Also the heat fixing apparatus of the film heating method is disclosed for example in the Japanese Patent Application Laid-Open Nos. 63-313182, 2-157878, 4-44075 to 4-44083, 4-204980 to 4-204984 and the like. A heat-resistant fixing film (fixing film) constituting a heating rotary member is slid frictionally and conveyed in contact with a heating member by means of a pressing rotary member (elastic roller), and a recording material (hereinafter also called transfer material) bearing an unfixed image is introduced into the close contacting nip formed by the heating member and the pressing rotary member across the heat-resistant fixing film and conveyed together with the heat-resistant fixing film. Thus the unfixed image is fixed as a permanent image on the transfer material by the heat transferred from the heating member through the heat-resistant film and the pressure from the pressurizing rotary member at the close contacting nip.
The heat fixing apparatus of the film heating method can save electric power consumption and can achieve a shortened waiting time (quick starting) since it can employ a linear heating member of a low heat capacity and a thin film of a low heat capacity.
However, in the conventional heat fixing apparatus of the film heating method described above, heat conductivity in a direction (hereinafter called longitudinal direction) perpendicular to the conveying direction of the recording material is poor because the heater constituting the heating member and the fixing film constituting the heating rotary member are both small in heat capacity. Consequently, in case of passing a recording material of a width smaller than the maximum size, there tends to result a significant temperature increase in a non-passing area of the recording material, leading to thermal damage to a supporting member for the heater, the film, the pressure roller etc. In order to prevent such thermal damage, it has therefore been necessary to decrease the throughput of the smaller-sized sheet.
Also in case of passing a wide recording sheet immediately after passing a smaller recording sheet, the hot offset phenomenon tends to occur in the non-passing area of the smaller recording sheet because the heater and the pressure roller are at a higher temperature only in such non-passing area. In order to prevent such phenomenon, it has been necessary to provide a pause time after passing the smaller-sized sheet, prior to the passing of the wide recording sheet.
In order to prevent such phenomenon, it has been proposed to divide the heat generating member of the heater into plural patterns and to vary the heat generating area according to the width of the recording material, but such method has not been practiced because of the following drawbacks.
In case of passing a narrow recording material, the temperature on the ceramic substrate becomes abruptly higher in the non-passing area immediately outside the sheet passing area. Therefore, in order to accommodate recording materials of various widths, it becomes necessary to independently control the power supply to the heat generating members of many kinds. Thus there are required a number of contact electrodes corresponding to the independent heat generating members, whereby the heater becomes not only extremely bulky but also the driving circuits for driving the heat generating members are required in an excessively large number and become unacceptably costly.
Also, in the heat fixing apparatus of the film heating method, the heat generating member has to be made wide in order to secure the sufficiently wide heat transmitting area, corresponding to the increasing process speed of the image forming apparatus. For this reason, in order to independently drive many heat generating members corresponding to various sizes of the recording material, the required substrate size increases with the increase of the process speed, thus resulting in an unacceptably high cost.
Also the heat generating member, having a shorter heat generating portion corresponding to a smaller-sized sheet, tends to show a large current because of the reduced resistance, thus eventually resulting a flickering phenomenon.
An object of the present invention is to provide an image heating apparatus capable of preventing the temperature increase in the sheet non-passing area without an excessive increase in the number of the heat generating members, the electrodes or the like.
Another object of the present invention is to provide an image heating apparatus and a heater capable of preventing the flickering phenomenon even when a small-sized recording material is used.
Still another object of the present invention is to provide an image heating apparatus comprising a heater having plural heat generating members provided on a long-shaped ceramic substrate and adapted for generating heat by means of a power supply and a film to be contacted with a recording material bearing an image thereon, wherein the plural heat generating members have different distributions of generated heat in the longitudinal direction of the substrate, and the film contacts a face of the heater opposite to the face thereof bearing the heat generating members, whereby the image on the recording material is heated by the heat from the heater via the film.
Still another object of the present invention is to provide an image heating apparatus comprising a heater having plural heat generating members provided on a long-shaped substrate and adapted for generating heat by means of a power supply and a film to be contacted with a recording material bearing an image thereon, wherein the plural heat generating members have different distributions of generated heat in the longitudinal direction of the substrate, and the heater has a high heat-conductive (thermal conductive) member provided on the heat generating members and the film contacts the high heat-conductive member whereby the image on the recording material is heated by the heat from the heater via the film.
Still another object of the present invention is to provide a heater comprising a long-shaped substrate, and first and second heat generating members provided on the substrate along the longitudinal direction thereof and adapted to generate heat by means of a power supply, wherein the length of the second heat generating member in the longitudinal direction of the substrate is smaller than that of the first heat generating member and the resistance value per unit length of the second heat generating member in the longitudinal direction of the substrate is larger than that of the first heat generating member, and an image heating apparatus provided with such heater.
Still other objects of the present invention, and the features thereof, will become fully apparent from the following description.